High frequency coupling device



Jan. 17, 1939. w. J. POLYDOROFF ,1 ,0

HIGH FREQUENCY COUPLING DEVICE Filed June 5, 1937 l N V E N TO R W1 40/1? 1 04 YDOROFf BMMIGM ATTO R N EY Patented Jan. 17, 1939 UNITED STATES2,144,029 men FREQUENCY COUPLING nsvroa Wladimir J. Polydorofl', JohnsonLaboratories poration of Illinois Wilmette, 111., asaignor to Inc.,Chicago, 111., a cor- Application June 3, 1931, Serial No. 140,239

16 Claims.

This invention relates to improvements in highfrequency resonantsystems, such, for example, as those which are generally employedbetween the output terminals of a first vacuum tube and the inputterminals of a second vacuum tube, and which are intended for operationat a particular frequency, or over a limited range of frequencies, forexample, in the intermediate-frequency am plifiers of radio receivers ofthe superheterodyne type.

Resonant systems of the above-mentioned type are of two general forms,one employing a single resonant circuit and the other employing two ormore coupled resonant circuits, each circuit having an inductor and acapacitor, one or both of which elements is adjustable to align thecircuits at a particular frequency or to permit varying the resonantfrequency of each circuit over a limited range.- The present inventionis particularly addressed to the second form.

Fixed air-core inductors shunted by adjustable tuning capacitors havebeen in common use in coupled resonant-circuit devices, the twoinductors being so arranged with respect to each other as to provide, atsome chosen frequency, a desired degree of inductive coupling betweenthe two resonant circuits. The usual type of adjustable "trimmer"capacitor, however, is appreciably affected by changes in temperature,and also variations in capacitance value with time, thus making itdifiicult if not impossible to maintain the desired performancecharacteristics in capacitively tuned fixed-inductance coupledresonantcircuit devices of the types now commonly em ployed.

The difficulty just described may be eliminated by employing fixedcapacitors and accomplishing tuning by the use of variable inductors,preferably oi the type employing movable ferromagnetic cores forinductance adjustment. In such a device, the resonant circuits may betuned by inductance variation to secure alignment at a desiredfrequency, and, once established, this alignment is substantiallypermanent.

In the design 01' devices of the class here under consideration. it isnecessary to first determine the frequency at which they are to beoperated, or the range of frequencies over which they are to beadjustable A particular device will. in general, be designed for use ata particular frequency, but will be adjustable and operative over asmall range of frequencies. When the device is adjusted to operate atthe particular frequency for which it was designed, either thecapacitance or the inductance will be varied to compensate fordeviations in the capacitances and inductances in the circuits withwhich the device is associated, in such a way as to bring the product ofthe total eilective capacitance and the total efiective inductance tothe correct value for the particular 5 frequency. Thus, if theinductance value in a particular case is high, the capacitance valuemust be correspondingly lower, and vice versa.

Under the circumstances just described, it now becomes possible inaccordance with the invention to design a device so that, for anyadjustment of its variable elements, it will produce the same resonantgain or the same selectivity characteristic, as may be desired.

Since modern radio receiving apparatus usually has adequate gain and inaddition has automatic gain control to compensate for any changes ingain or in the strength of the received signal, and since, on the otherhand, variations in the selectivity characteristic result in variationsin fidelity and may produce serious distortion, it is preferable todesign the device so that it will maintain the selectivity constant atthe desired value regardless oi the settings of its adjustable elements.

In general, there are two methods by which the over-all selectivity ofsuch a device may be kept substantially constant. In the first method,each of the circuits isso designed that its individual selectivity willremain constant, and the coupling between the two circuits is soarranged that it remains constant, regardless of adjustments in eitherof the circuits. In the second method, the individual selectivities ofthe circuits are allowed to vary but the coupling between the twocircuits is arranged to also vary in such a manner as to 85substantially compensate for the individual circuit variations. Thefirst method, and devices which enable it to be carried out, constitutethe subject matter of the present application. My co-pendingapplication, Serial Number 146,241, filed June 3rd, 1937, is addressedto the second method.

The condition to be fulfilled it each of the circuits is to haveconstant selectivity is that the ratio of its inductance to itsresistance, or L/R, shall remain constant for any position of theadjustments necessary to secure resonance at the intended frequency. If,in addition, the arrangement is such that the degree of coupling betweenthe two circuits remains constant for all positions of the adjustments,then the over-all selectivity will also remain constant.

It is an object of this invention to provide a high-frequency couplingdevice in which the coupling between the resonant circuits is reguiatedto compensate for the effect of adjustments of the individualresonant-circuit inductance values.

Another object of the invention is to provide a high- "equency couplingdevice in which substantially uniform selectivity may be secured,regardless of inductance adjustment, without resort to expensive orcomplicated arrangements.

United States Patent No. 2,066,777 to Harnett shows and describes acoupling device including two coupled resonant circuits which are tunedby inductance variation. The inductance of the windings is adjusted byvarying the spacing between the sections or pies of each winding, andthe arrangement is described as being such that the degree of couplingbetween the two resonant circuits is maintained constant, as theinductance values are changed by relative movement of the sections ofeither winding to tune the circuits to the desired frequency, in orderto achieve constant selectivity. It is essential, in the device of theHarnett patent, that the ratio of inductance to resistance, or L/R, ofthe individual circuits remain substantially constant throughout therange of their adjustability, if the degree of coupling between thecircuits remains constant and the selectivity of the complete device isto be substantially constant for operation at a given fixed frequency.The Harnett patent, however, fails to recognize the necessity formaintaining L/R constant in the individual circuits, nor does itdisclose how this result could be achieved.

Patent No. 2,066,777, above referred to, states that the principles ofthe invention may be practiced in various other suitable ways, as forexample, by changing the self-inductances of the individual windings byadjusting the relative positions of magnetic cores employed inconnection with the remote sections of the windings. The Harnett patentdoes not disclose, however, how these cores should be arranged toprovide substantially constant L/R of the individual circuits and thusuniform selectivity over the range of adjustability, nor is there anymention of the efiect which movement of the magnetic cores is bound tohave on the mutual inductance of the device. This efiect is appreciableand must be taken into consideration if the degree of coupling is to bemaintained substantially constant.

In accordance with the present invention, the inductors are so designedthat at the chosen frequency and regardless of any variation ofinductance, the ratio of inductance to resistance, L/R, is keptconstant. Each circuit then has constant selectivity throughout itsrange of adjustability. Additionally, in order to maintain the over-allselectivity of the two coupled circuits constant, the device may be sodesigned that the capacitive coupling between the circuits isnegligible, and as to maintain the inductive coupling coeficient inconstant, and this may be accomplished by providing that for any changein the inductance values, L1 or L2, of either of the inductors, themutual inductance Lm is changed in the same direction, these quantitiesbeing related in the well-known equation It is possible to use myimproved coupling de vice not only at a particular frequency as abovedescribed, but also at some other frequency within a limited range. Forexample, an intermediatefrequency coupling device in accordance with myinvention may be made to operate at any freaieaoae quency within itsrange of adjustability in order to secure the best possiblediscrimination against a particular local image-frequency interference.

If the frequency change were to be efiected solely by change ofinductance and it was desired to maintain the selectivity of the pair ofcoupled circuits substantially constant, one method of securing thisresult would be to so construct the inductances that the L/R ratio ofeach circuit remained constant throughout the range of frequencies andto so design the unit that the coupling coeflicient would be inverselyproportional to the frequency. The inductances would each be inverselyproportional to the square of the frequency, and the mutual inductance,therefore, would be inversely proportional to the cube oi the frequencyand hence would change more rapidly than the inductances. If such a unitwere to be associated with an amplifying tube, not only the selectivitybut also the amplification would remain constant throughout the range ofadjustability.

As will be clear from what is to follow, the coupling device of thepresent invention not only satisfies the conditions for constantselectivity at a'single frequency, but also produces 'approximatelyconstant selectivity and constant amplification over a range offrequencies.

My improved coupling device, therefore, although particularly adaptedfor operation at a single frequency, may also be used successfully fortuning over a range of frequencies or to some frequency other than theparticular frequency for which the device is primarily designed. Theindividual inductors are designed to maintain constant L/R at aparticular frequency and so, when used over a frequency range, theincrease in inductance for a lower frequency results in somewhatimproved L/R due to the relatively low losses in the ferromagnetic coresand to the decreasing resistance of the coils. This, however, closelycorresponds to conditions for maintaining the quantity Q=wL/R constant.For this case, as is known, the coupling coefiicient should remainconstant, which, as already pointed out, is a feature of my invention.My improved coupling device, therefore, may be tuned throughout a rangeof frequencies and provides a pair of coupled circuits withsubstantially constant coeflicient of coupling, and although theselectivity will be somewhat improved as the frequency decreases, thefact that the coefficient of coupling remains substantially constant isof particular advantage when critical couplings are employed, as isusually the case.

In addition to inductive coupling between the two circuits of thecoupling device due to mutual inductance, there may be some capacitivecoupling between the circuits. The capacitive coupling may either aid oroppose the inductive coupling, depending upon its phase with respect tothe phase of the inductive coupling. The degree of capacitive couplingmay be expressed as follows:

where Cm is the mutual capacitance and C1 and C2 are the capacitances inthe two circuits. Since Cm is usually very small compared with C1 and Ina particular coupling device, Cm depends upon the spacing and theconnections of the windings,

and C1 and Ca include not only the tuning capacitances but also theinput and output capacitances of the vacuum tubes which may beassociated with the device and the stray capacitance of the connectingleads.

United States Patent No. 2,06637'7, above referred to, also makes nomention of capacitive coupling between the two resonant circuits of itscoupling device, nor does the patent show any means for utilizing oreliminating it. If a coupling device is to provide uniform performanceat fixed frequency over a range of adjustability oi the circuitinductances and capacitances, it is necessary to take into considerationthe capacitive coupling which aids or opposes the inductive coupling.This is especially important in the case of highly eflicient circuitswhere the degree of coupling is very small. Movement of a portion ofeach winding, as shown in the Harnett patent, for example, is bound tohave an appreciable effect upon the capacitive coupling which existsbetween the two resonant circuits including the windings.

In one advantageous embodiment of my invention, I employ a combinationof inductive and capacitive couplings which are so correlated as tomaintain the degree of over-all coupling substantially constant at fixedfrequency as the circuit inductances and capacitances are varied. Inthis case, the capacitive coupling is arranged to aid the inductivecoupling, so that the total coupling coeflicient may be expressed asfollows:

Substituting Equations 1 and 3 in Equation 4, the result is:

m m k=:

1 2 1/ i 2 (5) From Equation 5, it is clear that the total couplingcoeflicient may be maintained substantially constant either bymaintaining each term of the right-hand side of the equation constant,or by so proportioning the inductive and capacitive couplings that thesecond term compensates for inconstancy of the first term.

If for instance the capacitances in each of the individual circuits arelower than they should he, say by about 10%, then the expression islikewise about 10% lower. Assuming that the accompanying decrease in themutual capacitance is negligibly small, the total capacitive coupling isabout 10% higher. Therefore in order that the total effective degree ofcoupling between the two circuits may remain unchanged and assuming knand kc to be substantially equal, the increase in the capacitivecoupling must be compensated by a corresponding (10%) decrease in theinductive coupling, when the inductances of the circuits are adjusted tore-establish resonance at the desired frequency.

At constant frequency, L1C1=Li'Ci' and L2C2=La'C2', the primesdesignating the changed values of inductance and capacitance in theindividual circuits, brought about by the unintentional changes ofcapacitance and the compensating changes of inductance to reestablishresonance at the desiredfrequency. Therefore a 10% decrease of the valuemust be accompanied by a 10% increase of the value V 1 to JUL. and ifthis increase in the inductance values of the individual circuits isperformed without noticeable change of the mutual inductance then thetotal efl'ective coupling will be the same in both cases, i. e.,

If, in addition, the inductance adjustment has been performed with acore which maintains the L/R ratio of the circuit constant as theinductance is changed, then the selectivity within and between thecircuits, and hence the over-all selectivity, remains substantiallyconstant, with no change in either the mutual inductance or the mutualcapacitance.

Thus the total coupling cofficient is maintained substantially constant,in spite of the fact that the mutual inductance does not vary in directproportion to the square root of the product of the individual circuitinductances. This advantageous utilization of capacitive coupling, in accordance with my invention, greatly simplifies the design of couplingdevices capable of providing substantially uniform selectivity over awide range-of adjustability.

The invention will be better understood by reference to the accompanyingdrawing, in which:

Fig. 1 shows a schematic wiring diagram of one form of the couplingdevice; and

Fig. 2 shows, partly in section, a preferred embodiment of a couplingdevice of the form of the invention shown in Fig. 1.

Referring to Fig. 1, vacuum tubes i and 2 are shown coupled by acoupling device contained within shield 3. The output circuit of vacuumtube I includes an input resonant circuit comprising serially connectedcoils 4 and 4a and shunt capacitor 8. Input coil 4 is provided with afixed ferromagnetic core 6. adjustable in its inductance value by meansof movable ferromagnetic core 1. Similarly, an output resonant circuitcomprising serially connected coils 4 and 4a and shunt capacitor 8 isconnected in the input circuit of vacuum tube 2. Output coil 4 isprovided with a fixed ferromagnetic core 6, and output coil 411 has anadjustable ferromagnetic core 1. By adjustment of cores I relatively tocoils 4a, the effective inductances of -the input and output resonantcircuits, respectively, may be varied. Input coils 4 and 4a in seriesare inductively coupled to output coils l and la in series. The degreeof capacitive coupling may be increased by employing a capacitor 20shown dotted between the high-potential terminals of the input andoutput circuits.

A preferred embodiment of my improved coupling device is shown in Fig.2, in which coils l and 4a are mounted on an insulating tube 5vertically disposed in shield can 3. Cores 6 are fixedly mounted insideof tube 5 in a desired relation to coils 4, respectively. An internallythreaded nut 9 rotates in insulating assembly plate I0, which is securedto shield can 3 by means of bolts II and nuts l2. Spring washer l3secures nut 9 in plate l0. Sleeve H, to which is secured tube 5 andguiding member 15, is externally threaded to engage the threads of nut9. Member l5 slidably engages bolts II and thus prevents rotation ofsleeve I4. Assembly plate l6, which is secured to the open end of shieldcan 3 by means of mounting spade bolts ll, supports lower core 1 infixed position by means of screw i8 inserted in rubber washer l9, towhich the core is attached. Thus rotation of nut 9 imparts a verticalmotion to tube 5 bearing lower coil 4a,

Input coil to is and the relative-position of lower coil 4a and lowercore 1 is therefore changed. Upper core 1' is similarly secured to anadjusting screw l8, which engages the internal threads of sleeve I4,

5 thus permitting adjustment of its position with 1 bly being movedrelatively to lower core I, so that in effect rotation of nut 9 merelyadjusts the position of lower core I relatively to lower coil 4a,producing no other change. It will be readily understood by thoseskilled in the mechanical arts 15 that other arrangements to produceidentical relative motions of the parts of the assembly can be devised.Since these alternative arrangements will be the exact electrical andmagnetic equivalents of the arrangements which I have de- 20 scribed,they are to be understood to lie within the scope of my invention.

-Figure 2 may be taken as illustrative of a practical design of theembodiment shown, for use at a frequency of 460 kilocycles, and isapproxi- 25 mately a full-size drawing of such a device. The

ferromagnetic cores 5, 6 and l, I may be, for example, in diameter andthe tube 5, upon which the coils 4, 4 and 4a, 4a are directly wound,

has a thin wall and provides a sliding fit for the 30 cores 6, 6 andI, 1. The coils 4, 4 and 4a, 4a are of the universal-wound type,preferably of so- -called Litz wire having a plurality of individuallyinsulated strands, and each pair may be wound to an inductance value ofapproximately 1300 35 microhenries with the cores fully inserted, and

may be adjusted to values approximately 20% lower by partial withdrawalof the cores.

The inner cores are adjusted to leave distances of about between themand the inner coils to leave a distance of 1%. The desired mutualinductance Lm may be secured by placing the outer coils from the innercoils. Such a selection of distances maintains a desired variation of Lmwhen the inductances are varied, and provides uniform operation of thecoupling device over the range of adjustability of the cores.

Fixed capacitors 8, which may have a value of 70 micromicrofarads, aresecured to members l5 and I6, respectively, and are connectedin shuntwith each serially connected pair of coils 4 and 4a to form the inputand output resonant circuits, respectively.

In operation, moving the cores 1 toward the cores 6 gradually closes theair gaps and thus 55 materially shortens the magnetic paths of the twogroups of coils 4 and 4a. This in turn not only increases the totalinductance in each circuit by a substantial amount, by virtue of theincreased linkages between the two coils in that v60 circuit, but alsoincreases the effective inductance of coils 4 as well as that of coils4a. By appropriate choice of the coils and cores, the resonant circuitsmay be made to have substantially constant L/R at constant frequency andwill have 65 nearly constant Q values over a range of fre- 75 theirassociated cores 1.

If the inductors are made in accordance with the disclosure of my UnitedStates Patent No. 1,982,690, the ferromagnetic core, when it is fully orpartially inserted in the associated coil, in-

creases the L/R and hence the Q of the coil. Thus the use of properlydesigned inductors employing magnetic cores provides not onlysubstantially constant selectivity throughout the range ofadjustability, but also an improved degree of selectivity over thatpreviously realized in coupling devices intended for the same purpose.This double advantage of employing magnetic cores in an inductance-tunedcoupling device is one of the features of the present invention.

According to one form of the invention, the mutual inductance isarranged to increase proportionately to increases in theindividualcircuit inductances in order to maintain the degree of inductivecoupling substantially constant. {According to the invention, the mutualinductance between the input and output resonant circuits may be made toincrease at the proper rate as the cores 1 are moved closer together byseveral simultaneous and cooperating effects. The increase in theindividual inductances of coils 4a due to the introduction of cores 1into them results in a greater contribution to the mutual inductance bythese outer coils. Additionally,

v the magnetic path for the flux linking the coils of the two circuitsis shortened by the movement of cores 1, with resultant strengthening ofthe mutual inductance.

It is within the scope of the invention to so design the coupling devicethat the mutual inductance increases at a rate which is notsubstantially equal to that required for maintaining a constant degreeof inductive coupling. If a higher rate of increase is desired, coils 4amay be made to have an inductance value somewhat greater than that ofcoils 4. If the outer coils 4a have less inductance than the inner coils4, the rate of change of mutual inductance will be lower than would berealized were the coils 4 and 4a of similar inductance. The mutualinductance may be supplemented by mutual capacitive reactance of theproper phase and magnitude to provide, in conjunction with the inductivecoupling, a desired degree of over-all coupling. If the windings are soconnected that the outer turns of coils 4 are at low high-frequencypotential, the capacitive coupling between the circuits is very low. Byconnecting the outer turns of coils 4 to the high-potential sides of thecircuits, however, an appreciable degree of capacitive coupling isobtained. This capacitive coupling may be still-further increased by theuse of capacitor 20 (see Fig. 1) connected between the high-potentialterminals of the two resonant circuits. Whether the capacitive couplingthus obtained aids or opposes the inductive coupling depends upontherelative winding directions of the two pairs of serially connected coils4 and 4a.

The scope of the invention is not limited to the embodiments shown inthe drawing and described herein, but includes as well such furthermodified forms as will occur to those skilled in the art and embodyingthe principles hereinabove disclosed.

Having thus described my invention, what I claim is:

' l. A high-frequency coupling device including plural inductance coilsand capacitors connected to provide a pair of coupled resonant circuits,and ferromagnetic cores adjustable relatively to said coils in such amanner that the selectivity of each of said circuits ismaintained-constant, said 9,144,029 coils and said cores being sodisposed that move-,

ment of said cores relative to said coils produces simultaneous andsubstantially proportional changes in the square root of the product ofthe eifective inductances of said circuits and in the mutual inductancebetween said circuits.

2. A high-frequency coupling device including plural inductance coilsand capacitors connected to provide a pair of coupled resonant circuits,and ferromagnetic cores adjustable relatively to said coils in such amanner that the selectivity of each of said circuits is maintainedconstant, said coils and said cores being so disposed that movement ofsaid cores into said coils increases the eiiective inductances in saidcircuits and increases the coupling between said circuits in such a wayas to maintain the selectivity of said coupling device substantiallyconstant.

3. A high-frequency coupling device including plural inductance coilsand capacitors connected to provide a pair,of coupled resonant circuits,and ferromagnetic cores adjustable relatively to said coils in such amanner that the selectivity of each of said circuits is maintainedconstant, said coils and said cores being so disposed that movement ofsaid cores relative to said coils alters the resonant frequency of saidcircuits while maintaining the degree of coupling between said circuitssubstantially constant.

4. A high-frequency coupling device including plural inductance coilsand capacitors connected to provide a pair of coupled resonant circuits,and ferromagnetic cores adjustable relatively to said coils in such amanner that the selectivity of each of said circuits is maintainedconstant, said coils and said cores being so disposed that movement ofsaid cores relative to said coils alters the effective inductance ofsaid circuits while maintaining the degree of coupling between saidcircuits substantially constant.

5. A high-frequency coupling device including two groups of inductancecoils, plural capacitors;

said coils and said capacitors beingconnected to provide a pair ofinductively coupled resonant circuits, ferromagnetic cores fixedlypositioned relatively to a first of said groups, and adjustableferromagnetic cores movable relatively to the second of said groups,said coils and said cores being so disposed that movement of saidadjustable cores relative to said second group produces simultaneous and.substantially proportional changes in the square root of the productofthe effective inductances of said circuits and in the mutualinductance between said circuits.

6. A high-frequency coupling device including two groups of inductancecoils, plural capacitors, said coils and said capacitors being connectedto I provide a pair of coupled resonant circuits, ferromagnetic coresfixedly positioned relatively to a first of said groups, and adjustableferromagnetic cores movable relatively to the second of said groups,said coils and said cores being so disposed that movement of saidadjustable cores into said second group increases the efi'ectiveinductances in said circuits and increases the coupling between saidcircuits in such a way as to maintain the selectivity of said couplingdevice substantially constant.

7. A high-frequency coupling device including two groups of inductancecoils, plural capacitors, said coils and said capacitors being connectedto provide a pair of coupled resonant circuits, ferromagnetic coresfixedly positioned relatively to a first of said groups, and adiustableferromagnetic cores movable relatively to the second of said,

groups, said coils and said cores being so disposed that movement ofsaid adjustable cores relative to said second group alters the resonantfrequency of said circuits while coupling between said onant.

8. A high-frequency coupling device including two groups of inductancecoils, plural capacitors, said coils and said capacitors being connectedto provide a pair of coupled resonant circuits, ferromagnetic coresfixedly positioned relatively to a first of said groups, and adjustableferromagnetic cores movable relatively to the second of said groups,said coils and saidcores being so disposed that movement of saidadjustable cores relative to said second group alters the effectiveinductances of said circuits while maintaining the degree of couplingbetween said circuits subcircuits substantially res- -stantiallyresonant.

9. A high-frequency coupling device including first and second resonantcircuits, first and second inductive windings in each of said circuits,ferromagnetic cores fixedly positioned within said first windings, andadjustable ferromagnetic cores positioned within but movable relativelyto said second windings, said windings and said cores being so disposedthat movement of said adjustable cores relative to said second windingsproduces simultaneous and substantially proportional changes in thesquare root of the product of the eifective inductances ofsaid circuitsand in the mutual inductance between said circuits.

10. A high-frequency coupling device including first and second resonantcircuits, first and secand inductive windings in each of said circuits,ferromagnetic cores fixedly positioned within said first windings, andadjustable ferromagnetic cores positioned within but movable relativelyto said second windings, said windings and said cores being so disposedthat movement of said adjustable cores into said second windingsincreases the efi'ective inductances in said circuits and increases thecoupling between said circuits in such a way as to maintain theselectivity of said coupling device substantially constant.

11. A high-frequency coupling device including first and second resonantcircuits, first and second inductive windings in each of said circuits,ferromagnetic cores fixedly positioned within said first windings, andadjustable ferromagnetic cores positioned within but movable relativelyto said second windings, said windings and said cores being so disposedthat movement of said adjustable cores relative to said second windingsalters the resonant frequency of said circuits while maintaining thedegree of coupling between said circuits substantially constant.

12. A high-frequency coupling device including first and second resonantcircuits, first and second inductive windings in each of said circuits,ferromagnetic cores fixedly positioned within said first windings. andadjustable ferromagnetic cores positioned within but movable relativelyto said second windings, said windings and said cores being so disposedthat movement of said adjustable cores relative to said second windingsalters the effective inductances of said circuits while maintaining thedegree of coupling between said circuits substantially constant. 7

13. A high-frequency coupling device including first and second resonantcircuits, first and second inductive windings in each of said circuits,ferromagnetic cores fixedly positioned within said first windings, andadjustable ferromagnetic cores positioned within but movable relativelyto said maintaining the degree of second windings, said windings andsaid cores being coaxiaily arranged and so disposed that movement ofsaid adjustable cores relative to said second windings producessimultaneous and substantially proportional changes in the square rootof the product of the eflective inductances positioned within butmovable relatively to said second windings, said windings and said coresbeing coaxiaily arranged and so disposed that movement of saidadjustable cores into said second windings increases the eiiectiveinductances in said circuits and increases the coupling between saidcircuits in such a way as to maintain the selectivity of said couplingdevice substantially constant.

15. A high-frequency coupling device including first and second resonantcircuits, first and second inductive windings in each of said circuits.

ferromagnetic cores fixedly positioned within said first windings, andadjustable ferromagnetic cores positioned within but movable relativelyto said second windings, said windings and said cores being coaxiailyarranged and so disposed that movement of said adjustable cores relativeto said second windings alters the resonant frequency of said circuitswhile maintaining the degree of coupling between said circuitssubstantially constant.

16. A high-frequency coupling device including first and second resonantcircuits, first and second inductive windings in each of said circuits,ferromagnetic cores fixedly positioned within said first windings, andadjustable ferromagnetic cores positioned within but movable relativelyto said second windings, said windings and said cores being coaxiailyarranged and so disposed that movement of said adjustable cores relativeto said second windings alters the efifective induct- WLADIMIR J.POLYDOROFF. 25

